The inventive arrangements relate generally to substrate mounted antennas, and more particularly an improved arrangement of a loop antenna and associated feed structure.
RF circuits, transmission lines, and antenna elements are commonly manufactured on specially designed substrate boards. For the purposes of these types of circuits, it is important to maintain careful control over impedance characteristics. If the impedance of different parts of the circuit do not match, this can result in inefficient power transfer, unnecessary heating of components, and other problems. Electrical length of transmission lines and radiators in these circuits can also be a critical design factor.
Two critical factors affecting the performance of a substrate material are dielectric constant (sometimes called the relative permittivity or xcex5r) and the loss tangent (sometimes referred to as the dissipation factor). The relative permittivity determines the speed of the signal in the substrate material, and therefore the electrical length of transmission lines and other components implemented on the substrate. The loss tangent characterizes the amount of loss that occurs for signals traversing the substrate material. Losses tend to increase with increases in frequency. Accordingly, low loss materials become even more important with increasing frequency, particularly when designing receiver front ends and low noise amplifier circuits.
Printed transmission lines, passive circuits and radiating elements used in RF circuits are typically formed in one of three ways. One configuration known as microstrip, places the signal line on a board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar except that the signal line is covered with a dielectric substrate material. In a third configuration known as stripline, the signal line is sandwiched between two electrically conductive (ground) planes. Ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is equal to {square root over (L1/C1)} where L1 is the inductance per unit length and C1 is the capacitance per unit length. The values of L1 and C1 are generally determined by the physical geometry and spacing of the line structure as well as the permittivity of the dielectric material(s) used to separate the transmission line structures. Conventional substrate materials typically have a permeability of approximately 1.0.
In conventional RF design, a substrate material is selected that has a relative permittivity value suitable for the design. Once the substrate material is selected, the line characteristic impedance value is exclusively adjusted by controlling the line geometry and physical structure.
One problem encountered when designing microelectronic RF circuitry is the selection of a dielectric board substrate material that is optimized for all of the various passive components, radiating elements and transmission line circuits to be formed on the board. In particular, the geometry of certain circuit elements may be physically large or miniaturized due to the unique electrical or impedance characteristics required for such elements. For example, many circuit elements or tuned circuits may need to be an electrical xc2xc wave. Similarly, the line widths required for exceptionally high or low characteristic impedance values can, in many instances, be too narrow or too wide for practical implementation for a given substrate. Since the physical size of the microstrip or stripline is inversely related to the relative permittivity of the dielectric material, the dimensions of a transmission line can be affected greatly by the choice of substrate board material.
Still, an optimal board substrate material design choice for components such as antenna feed circuitry may be inconsistent with the optimal board substrate material for other components, such as antenna elements. Moreover, some design objectives for a circuit component may be inconsistent with one another. For example, it may be desirable to reduce the size of an antenna element. In the case of a dipole, this could be accomplished by selecting a board material with a relatively high permittivity. However, the use of a dielectric with a higher relative permittivity will generally have the undesired effect of reducing the radiation efficiency of the antenna.
From the foregoing, it can be seen that the constraints of a circuit board substrate having selected relative dielectric properties often results in design compromises that can negatively affect the electrical performance and/or physical characteristics of the overall circuit. An inherent problem with the conventional approach is that, at least with respect to conventional circuit board substrate, the only control variable for line impedance is the relative permittivity. This limitation highlights an important problem with conventional substrate materials, i.e. they fail to take advantage of the other factor that determines characteristic impedance, namely L1, the inductance per unit length of the transmission line.
Conventional circuit board substrates are generally formed by processes such as casting or spray coating which generally result in uniform substrate physical properties, including the dielectric constant. Accordingly, conventional dielectric substrate arrangements for RF circuits have proven to be a limitation in designing circuits that are optimal in regards to both electrical and physical size characteristics.
The invention concerns a printed circuit antenna with broadband input coupling. An elongated conductive antenna element arranged in the form of a loop is disposed on a dielectric substrate formed on a ground plane. The antenna element has first and second adjacent end portions separated by a gap. The second end portion is connected to the ground plane.
An input coupler is provided for matching an input impedance of the antenna to the antenna feed circuitry. The input coupler can comprise a conductive line disposed on the substrate adjacent to the antenna element. The conductive line is separated from the antenna element by a coupling space for coupling to the antenna element an input signal applied to the input coupler. The conductive line extends parallel to a portion of the antenna element including the first end portion. According to a preferred embodiment, the input coupler is disposed on a portion of the substrate within a perimeter defined by the antenna element. The arrangement has the advantage of having an input impedance that is relatively insensitive to adjustments affecting the antenna center frequency.
According to one aspect of the invention, a first region of the substrate comprising the coupling space has a permittivity that is different from the permittivity of a second region of the substrate on which is disposed the antenna element. Independently controlling the permittivity of the substrate in the coupling space allows greater control over capacitive coupling and therefore greater control over input impedance matching.
According to another aspect of the invention a region of the substrate on which the input coupler is disposed has a relative permeability that is smaller than the relative permeability of the second region of the substrate on which is disposed the antenna element. For example, the relative permeability of the second region can be greater than 1.
According to a further aspect of the invention, the antenna element can be divided into a plurality of elongated conductive segments. Each segment has adjacent end portions separated by a third characteristic region of the substrate. The third characteristic region of the substrate can have a permittivity that is larger than a permittivity of the second characteristic region of the substrate on which is disposed the elongated conductive segments. The gaps between the segments function as capacitors that can be adjusted in value by controlling the permittivity in the third characteristic region.